When tuning receivers, and particularly FM receivers, the overall high amplification causes noise to occur between tuning positions in which a particular station is precisely tuned. This inter-station tuning noise is disagreeable. When a transmitter has been tuned by a receiver, the S-shaped discriminator curve of the FM demodulator results in a low-frequency compensation, the basic frequency of which is dependent on the speed of tuning, that is, on the speed of changing the tuned circuit from detuned to tuned--and, if continued, to again detuned position.
Additional low-frequency noise disturbances occur due to locking of phase-locked loops (PLLs) mixing oscillator systems. The tuning voltges which are applied in such systems to frequency determining tuning diodes of the oscillators are modulated with the voltage which occurs during the tuning. This voltage will be presented to the FM demodulator of the receiver and will be reproduced as frequency modulation to the demodulation.
The inter-station tuning noise as well as the various compensation and leveling processes which occur result in undesirable interference noise, which can be heard by the user. Inter-station tuning noise can be suppressed in accordance with known circuit arrangements, usually referred to as muting arrangements and circuits. In known muting circuits, the audio frequency path has a gate circuit or switching circuit coupled therein which, during the tuning itself, blocks reproduction of audio output from the receiver for some predetermined period of time. The closing of this gate, which means interrupting the audio path to a reproducer, may be controlled by timing circuits with different time constants for first closing the gates and then opening the gates to permit transmission of audio output. The time constants upon opening the gates are usually substantially longer than those for closing the gates in order to mute even the lowest frequency tuning noises.
If a receiver is tuned very rapidly, for example upon a rapid transition from one transmitter to another, the scanning times or clocking times for the gate become so short that the lowest frequency noise or compensation noises will occur for a period of time longer than the scanning time. Known mating circuits, thus, can no longer carry out mating of inter-station tuning noises efficiently and effectively under such rapid tuning conditions. Rapid tuning may, for example, occur manually or in an automatic search mode of operation of a receiver.